feat(frontend): add React SPA with chat, auth, and admin pages

- Vite 6 + React 19 + TypeScript + Tailwind 4 + Zustand 5
- Login page with JWT auth
- Chat interface: MessageList, ChatInput, ChatContainer
- WebSocket hook with auto-reconnect and exponential backoff
- Sidebar with new chat button + session navigation
- Admin dashboard placeholder
- 57 modules, 242KB JS, TypeScript zero errors

frontend/web/src/components/chat/ChatContainer.tsx

@@ -0,0 +1,47 @@
+import { useEffect, useRef } from 'react'
+import { useChatStore } from '../../store/chatStore'
+import { useWebSocket } from '../../hooks/useWebSocket'
+import MessageList from './MessageList'
+import ChatInput from './ChatInput'
+
+export default function ChatContainer() {
+  const { send } = useWebSocket()
+  const { messages, isStreaming, currentSessionId, inputText, setInput, addMessage, setStreaming } = useChatStore()
+  const bottomRef = useRef<HTMLDivElement>(null)
+
+  useEffect(() => {
+    bottomRef.current?.scrollIntoView({ behavior: 'smooth' })
+  }, [messages])
+
+  const handleSend = () => {
+    if (!inputText.trim() || !currentSessionId) return
+
+    const userMsg = {
+      id: crypto.randomUUID(), session_id: currentSessionId,
+      role: 'user' as const, content: inputText, created_at: new Date().toISOString(),
+    }
+    addMessage(userMsg)
+    setStreaming(true)
+
+    // Add placeholder for assistant response
+    addMessage({ id: crypto.randomUUID(), session_id: currentSessionId, role: 'assistant', content: '', created_at: new Date().toISOString() })
+
+    send({ type: 'message', content: inputText, session_id: currentSessionId, timestamp: Date.now() })
+    setInput('')
+  }
+
+  return (
+    <div className="flex flex-col h-full">
+      <div className="flex-1 overflow-auto px-4 py-4">
+        <MessageList messages={messages} />
+        <div ref={bottomRef} />
+      </div>
+      <ChatInput
+        value={inputText}
+        onChange={setInput}
+        onSend={handleSend}
+        disabled={isStreaming}
+      />
+    </div>
+  )
+}